Novel copolymer, photoresist composition , and process for forming resist pattern with high aspect ratio

ABSTRACT

A novel copolymer includes a repeating unit (B) derived from an unsaturated carboxylic anhydride, a repeating unit (C) represented by Formula (II), and a repeating unit (D) represented by Formula (III).

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a chemically amplifiedphotoresist composition that is excellent in transparency to light withwavelengths in the deep ultraviolet region (deep UV light) such as KrFor ArF laser light, and exhibits high sensitivity and definition, to anovel copolymer that is suitable for the preparation of the photoresistcomposition, and to a process for the formation of a resist pattern witha high aspect ratio. Specifically, the present invention relates to achemically amplified photoresist composition that is suitable for aprocess for the formation of a resist pattern with a high aspect ratiousing a silylation agent, and to a process for the formation of a resistpattern with a high aspect ratio.

[0003] 2. Description of the Related Art

[0004] To form an ultrafine resist pattern of not more than 0.35 μm,specifically of not more than 0.25 μm, a variety of highly sensitivephotoresist compositions having satisfactory transparency to deep UVlight such as KrF or ArF laser light have been reported.

[0005] Among them, “chemically amplified” photoresist compositionscontaining a substance carrying an acid-decomposable group, and aphotosensitive acid generator are predominant. However, demands havebeen made to further improve the sensitivity and definition of thesechemically amplified photoresist compositions.

[0006] Additionally, the use of this type of photoresist compositionsalone cannot significantly form a resist pattern with a good shape,especially a hole-type resist pattern with a high aspect ratio and agood shape. As a possible solution to this problem, a technique has beenreported in which a second film or coating composed of a silylationagent is formed on the surface of a resist pattern, and using thissecond film, a resist pattern with a high aspect ratio is formed.

[0007] This technique using a silylation agent is reported, for example,in Microelectronic Engineering 11 (1990) 531-534 (Reference 1), and isperformed substantially in the following manner.

[0008] Initially, a resist film (bottom resist) is formed on asubstrate, and this bottom resist is resistant to etching treatment forthe substrate. Another resist film is then formed on the bottom resist,using a photoresist composition containing, for example, a polymerhaving a repeating unit represented by Formula (V) below, and the formedresist film is selectively patterned by exposure and developing tothereby form a resist pattern.

[0009] Next, a solution of, for example, a compound represented byFormula (VIII) below (silylation agent) is applied onto the resistpattern and is then rinsed to thereby form a silylation coating on theresist pattern, and the resulting silylation coating is resistant tocorrosion induced by oxygen plasma etching.

[0010] The underlayer bottom resist is removed by etching using, as amask, the resist pattern carrying the silylation coating to thereby forma resist pattern with a high aspect ratio.

[0011] Such photoresist compositions that can be applied to this processcomprising the above operations must essentially react with a silylationagent to form a silylation coating and must have high sensitivity anddefinition.

[0012] Japanese Patent Laid-Open No. 5-11450 (Reference 2) discloses, asthe aforementioned photoresist compositions, a photoresist compositioncontaining a polymer and a photosensitive acid generator, which polymercomprises a (meth)acrylic tert-butyl ester group and a maleic anhydridefunctional group, and a photoresist composition containing a polymer anda photosensitive acid generator, which polymer comprises a (meth)acrylictert-butyl ester group, a maleic anhydride functional group, and anallyltrimethylsilane group.

[0013] These compositions have satisfactory transparency to deep UVlight and have high sensitivity, but their definition must be furtherimproved.

[0014] Japanese Patent Laid-Open No. 11-212265 (Reference 3) discloses aphotoresist composition comprising a polymer and a photosensitive acidgenerator, which polymer has a repeating unit represented by Formula(III) and a repeating unit represented by Formula (VII):

[0015] wherein R¹ is a hydrogen atom or a methyl group; and R² is analkyl group having from 1 to 4 carbon atoms.

[0016] Even this composition must be further improved in sensitivity anddefinition.

[0017] Additionally, when this composition is applied to the processusing a silylation agent, it cannot significantly react with thesilylation agent to thereby fail to form a silylation coating, and ahole pattern with a high aspect ratio cannot be significantly obtained.

SUMMARY OF THE INVENTION

[0018] Accordingly, an object of the present invention is to provide achemically amplified photoresist composition that has satisfactorytransparency, high sensitivity and definition in the field ofphotolithography using a deep UV light source such as KrF or ArF laser,and to provide a novel copolymer that is suitable for the preparation ofthe photoresist composition.

[0019] Another object of the present invention is to provide achemically amplified photoresist composition that is suitable for aprocess for the formation of a resist pattern with a high aspect ratiousing a silylation agent, and to provide a process for the formation ofa resist pattern with a high aspect ratio.

[0020] After intensive investigations, the present inventors have foundthat the above objects can be achieved by the use of novel copolymershaving specific repeating units.

[0021] Specifically, the present invention provides, in an aspect, anovel copolymer including a repeating unit (B) derived from anunsaturated carboxylic anhydride, a repeating unit (C) represented byFormula (II), and a repeating unit (D) represented by Formula (III):

[0022] wherein R¹ is a hydrogen atom or a methyl group; and R² is analkyl group having from 1 to 4 carbon atoms.

[0023] In the novel copolymer, repeating unit (B) is preferably arepeating unit (B-1) derived from an unsaturated cyclic carboxylicanhydride.

[0024] Repeating unit (B-1) is preferably a unit (B-2) represented byFormula (V):

[0025] Alternatively, repeating unit (B-1) is preferably a unit (B-3)represented by Formula (XVI):

[0026] In the novel copolymer, repeating unit (D) may be a unit (D-1)represented by Formula (VI):

[0027] The content of repeating unit (B) is preferably equal to or morethan 15% and equal to or less than 60% of all repeating unitsconstituting the novel copolymer.

[0028] The content of repeating unit (C) is preferably equal to or morethan 10% and equal to or less than 40% of all repeating unitsconstituting the novel copolymer.

[0029] The content of repeating unit (D) is preferably more than 0% andequal to or less than 40% of all repeating unit constituting the novelcopolymer.

[0030] The novel copolymer may have a weight average molecular weight(Mw) in terms of polystyrene of from 7000 to 30000 and amolecular-weight distribution (Mw/Mn, where Mn is a number averagemolecular weight) of equal to or less than 3.5.

[0031] In another aspect, the present invention provides a photoresistcomposition including the novel copolymer, a photosensitive acidgenerator, and an organic solvent.

[0032] In the photoresist composition, the photosensitive acid generatoris preferably a triphenylsulfonium-based onium salt.

[0033] The organic solvent is preferably propylene glycol monomethylether acetate (PGMEA).

[0034] In addition and advantageously, the present invention provides aprocess for forming a resist pattern with a high aspect ratio, whichprocess includes the steps of (a) applying a first resist on a substrateand drying the applied first resist to thereby form a first resistlayer, applying the photoresist composition the first resist layer anddrying the applied photoresist composition to thereby form a secondresist layer; (b) exposing the second resist layer to imaging radiation,subjecting the exposed second resist layer to a heat treatment, anddissolving and removing exposed portions or unexposed portions of thesecond resist layer by developing in an alkaline aqueous solution tothereby form a resist pattern; (c) applying a silylation agent onto theformed resist pattern, rinsing the applied resist pattern to therebyenlarge the resist pattern and to form a silylation coating on theresist pattern, which silylation coating is resistant to corrosioninduced by oxygen-containing plasma etching; and (d) etching the firstresist layer under the second resist layer with oxygen-containing plasmaby using, as a mask, the enlarged resist pattern carrying the silylationcoating.

DETAILED DESCRIPTION OF THE INVENTION

[0035] [Novel Copolymers]

[0036] Novel copolymers of the present invention can be syntheticallyobtained by a known polymerization reaction using specific comonomers.Repeating units (B), (C) and (D) (hereinafter briefly referred to as“Ingredients (B), (C) and (D)”) will be illustrated in further detailbelow, as constitutional comonomers for the preparation of the novelcopolymers.

[0037] [Ingredient (B)]

[0038] Ingredient (B) constituting the invented copolymers is arepeating unit derived from an unsaturated carboxylic anhydride and isan essential unit, since the repeating unit serves to react with asilylation agent (e.g., a siloxane polymer having an amino group) tothereby form a silylation coating in the process using the silylationagent.

[0039] Comonomers corresponding to Ingredient (B) are not specificallylimited and include, for example, unsaturated carboxylic anhydridesdescribed in Japanese Patent Laid-Open Nos. 2-282746, 2-308255,2-308256, 5-9231,2-11450, 5-11456, and 11-212265. Such unsaturatedcarboxylic anhydrides include, for example, compounds represented byFormulae (IX) and (X), and other unsaturated acyclic carboxylicanhydrides; and compounds represented by Formulae (XI), (XII), (XIII),(XIV), and (XV), and other unsaturated cyclic carboxylic anhydrides:

[0040] wherein R⁴ is an alkyl group having from 1 to 5 carbon atoms oran aryl group; and R⁵ is a hydrogen atom or an alkyl group having from 1to 4 carbon atoms,

[0041] wherein each of R⁵ and R⁶ is independently a hydrogen atom or analkyl group having from 1 to 4 carbon atoms,

[0042] wherein R⁷ is a hydrogen atom or an alkyl group having from 1 to4 carbon atoms,

[0043] wherein each of R⁸ and R⁹ is independently a hydrogen atom or analkyl group having from 1 to 4 carbon atoms,

[0044] wherein R¹⁰ is a hydrogen atom or an alkyl group having from 1 to4 carbon atoms.

[0045] Among these comonomers, maleic anhydride is typically preferablefor its high reactivity with a silylation agent and satisfactorytransparency to deep UV light such as KrF or ArF laser light. PreferredIngredients (B) for use in the invented copolymers are repeating units(B-2) and (B-3) represented by Formulae (XI) and (XV) which are derivedfrom unsaturated cyclic carboxylic anhydrides.

[0046] [Ingredient (C)]

[0047] Ingredient (C) constituting the invented copolymers is arepeating unit represented by Formula (II) and serves to enhancereactivity with a silylation agent (e.g., a siloxane polymer having anamino group) in the process using the silylation agent. Additionally,the resulting photoresist composition obtained by using a copolymercontaining Ingredient (C) has improved sensitivity and definition.

[0048] A comonomer corresponding to Ingredient (C) isallyltrimethylsilane.

[0049] [Ingredient (D)]

[0050] Ingredient (D) constituting the invented copolymers is arepeating unit represented by Formula (III), which has an2-alkyl-2-adamantyl group as an acid-decomposable group and issatisfactory in transparency to deep UV light such as KrF or ArF laserlight. The substituent R¹ in the formula is a hydrogen atom or a methylgroup, and R² is an alkyl group having from 1 to 4 carbon atoms. Theresulting photoresist composition prepared by using a copolymercontaining Ingredient (D) has further improved sensitivity anddefinition.

[0051] Comonomers corresponding to Ingredient (D) include, for example,2-methyl-2-adamantyl acrylate, 2-ethyl-2-adamantyl acrylate,2-propyl-2-adamantyl acrylate, 2-butyl-2-adamantyl acrylate,2-methyl-2-adamantyl methacrylate, 2-ethyl-2-adamantyl methacrylate,2-propyl-2-adamantyl methacrylate, and 2-butyl-2-adamantyl methacrylate.Among them, 2-methyl-2-adamantyl methacrylate is typically preferred toyield higher sensitivity and definition of the resulting photoresistcomposition. These comonomers corresponding to Ingredient (D) can besynthetically obtained by known processes as described in, for example,Japanese Patent Laid-Open No. 11-212265. For example, these comonomerscan be obtained by an esterification reaction of an2-alkyl-2-adamantanol with acryloyl chloride or methacryloyl chloride.

[0052] [Contents of Ingredients (B), (C) and (D)]

[0053] The content of Ingredient (B) preferably falls within a rangefrom 15% to 60% inclusive, and more preferably from 35% to 50%inclusive, of all the repeating units constituting the novel copolymer.

[0054] If the content of Ingredient (B) is less than 15%, the reactivitywith the silylation agent may be insufficient, and if it exceeds 60%,Ingredient (B) may be readily converted into a dicarboxylic acid moietyby a reaction with moisture in the air or in the photoresist film tothereby vary the composition of the copolymer.

[0055] The content of Ingredient (C) preferably falls within a rangefrom 10% to 40% inclusive, and more preferably from 20% to 30%inclusive, of all the repeating units constituting the novel copolymer.

[0056] If the content of Ingredient (C) is less than 10%, the reactivitywith the silylation agent may not be sufficiently enhanced and thesensitivity and definition may not be satisfactorily improved. If itexceeds 40%, the definition and focal depth range properties may bedecreased.

[0057] The content of Ingredient (D) preferably falls within a range ofmore than 0% and equal to or less than 40%, and more preferably from 5%to 30% inclusive, of all the repeating units constituting the novelcopolymer.

[0058] If the content of Ingredient (D) is 0%, the resulting photoresistcomposition may not have satisfactory sensitivity and definition, and ifit exceeds 40%, the shape of resist pattern and focal depth rangeproperties may be deteriorated.

[0059] The most preferred embodiment is a copolymer comprising allIngredients (B), (C) and (D).

[0060] [Polymerization Reaction]

[0061] The invented novel copolymers can be synthetically obtained bysubjecting the individual comonomers to a polymerization reaction with apolymerization initiator.

[0062] Such polymerization initiators include, but are not limited to,those described in, for example, Japanese Patent Laid-Opens No. 5-11450and No. 11-212265, such as azobisisobutyronitrile anddimethyl-2,2-azoisobisbutyrate. Among them, azobisisobutyronitrile istypically preferred, since this compound can enhance the polymerizationreaction to thereby yield a copolymer having narrow molecular-weightdistribution.

[0063] An example of the production process of the invented novelcopolymers will be schematically illustrated below, but the invention isnot limited thereto.

[0064] (Step 1)

[0065] Initially, the material comonomers, polymerization initiator, andadditives are added to an organic solvent (e.g., ethyl acetate) in areactor and are dissolved in the organic solvent by stirring at roomtemperature for several ten minutes.

[0066] (Step 2)

[0067] The resulting mixture is heated to a temperature from about 60°C. to about 75° C., and is stirred at this temperature for several tenhours.

[0068] (Step 3)

[0069] After the completion of stirring, heating procedure is ceased,and the reaction mixture is cooled to about 30° C. and is then addeddropwise to a poor solvent such as petroleum benzine or 2-propanol withstirring to thereby precipitate a copolymer.

[0070] (Step 4)

[0071] The precipitated copolymer is rinsed with several portions of,for example, petroleum benzine and is then dried in a vacuum dessicatorset at several ten degrees Celsius to thereby yield a target copolymer.

[0072] The invented copolymer has a weight average molecular weight (Mw)in terms of polystyrene of preferably from about 7000 to about 30000 andmore preferably from about 10000 to about 20000. If the averagemolecular weight (Mw) in terms of polystyrene is less than 7000, thecopolymer may have low solubility in alcohols, and if it exceeds 30000,the resulting photoresist composition may have low definition.

[0073] The copolymer has a molecular-weight distribution (Mw/Mn) ofpreferably equal to or less than 3.5 and more preferably equal to orless than 2.5. If the molecular-weight distribution (Mw/Mn) exceeds 3.5,the resulting photoresist composition may have insufficient definition.

[0074] [Photoresist Composition]

[0075] Photoresist compositions according to the present invention canbe prepared by mixing the invented novel copolymer, a photosensitiveacid generator, and an organic solvent. The content of the novelcopolymer is preferably from 1 to 30% by weight and more preferably from3 to 10% by weight relative to the total weight of the composition.

[0076] [Photosensitive Acid Generator]

[0077] When the invented photoresist composition is used for theformation of a resist pattern, the photosensitive acid generatorliberates an acid in exposed portions in the step of exposing aphotoresist film to imaging radiation. Additionally, a subsequent heattreatment permits an “acid-decomposable group” (e.g., an2-alkyl-2-adamantyl group) in the copolymer skeleton of the compositionto eliminate, which acid-decomposable group is decomposed by thecatalytic reaction of the acid and becomes soluble in alkali.Consequently, exposed portions of the resist film are dissolved in adeveloper solution, an alkaline aqueous solution, to thereby yield aresist pattern.

[0078] Photosensitive acid generators for use in the present inventioninclude, but are not specifically limited to, those described in, forexample, Japanese Patent Laid-Open Nos. 5-11450 and 11-212265, such asonium salt compounds, organohalogen compounds, sulfone compounds, andsulfonate compounds. Among them, onium salt-based photosensitive acidgenerators are preferred, since the resulting photoresist compositionshave high sensitivity and can form a resist pattern with a good shape,of which triphenylsulfonium-based onium salts are typically preferredfor high sensitivity.

[0079] Such triphenylsulfonium-based onium salts include, for example,the following compounds represented by Formulae (E-1) to (E-3):

[0080] The amount of the photosensitive acid generator is preferablyfrom 0.01 to 3.0% by weight and more preferably from 0.03 to 0.8% byweight relative to the total weight of the composition.

[0081] [organic Solvent]

[0082] Organic solvents for use in the invention include, but are notspecifically limited to, acetone, methyl ethyl ketone, cyclohexanone,methyl isoamyl ketone, 2-heptanone, and other ketones; ethylene glycol,propylene glycol, diethylene glycol, ethylene glycol monoacetate,propylene glycol monoacetate, diethylene glycol monoacetate, ormonomethyl ethers, monoethyl ethers, monopropyl ethers, monobutylethers, or monophenyl ethers of these compounds, and other polyhydricalcohols and derivatives thereof; dioxane and other cyclic ethers; andethyl lactate, methyl acetate, ethyl acetate, butyl acetate, methylpyruvate, ethyl pyruvate, methyl methoxypropionate, ethylethoxypropionate, and other esters. Each of these organic solvents canbe used alone or in combination.

[0083] Among them, propylene glycol monomethyl ether acetate (PGMEA) ispreferred, since this solvent has good coating performance, lowhygroscopicity, and a high boiling point. Solvents having highhygroscopicity are not desirable, since the resulting photoresistcomposition takes in moisture in the air to thereby deteriorate thecopolymer. Additionally, solvents having a low boiling point are alsoundesirable, since the amount of solvent remained in the formedphotoresist film is low and the photosensitive acid generator does notefficiently diffuse in the procedure of, for example, post-exposurebaking (PEB).

[0084] [Other Additives]

[0085] The invented photoresist compositions may further comprise knownor conventional additives according to necessity, within ranges notadversely affecting the objects of the present invention. Such additivesinclude, for example, dissolution inhibitors, other resins, stabilizers,organic polymeric compounds, thermopolymerization inhibitors,sensitizers, improvers for adhesion to substrates, surfactants, anddyes.

[0086] The invented photoresist compositions are suitable as positivephotoresist compositions, but can be used as negative photoresistcompositions for the formation of negative resist patterns, byincorporating acid-crosslinkable materials (substances which arecrosslinked by action of an acid generated from a photosensitive acidgenerator) into the photoresist compositions.

[0087] The invented composition is preferably used, for example, in thefollowing manner. Initially, a solution of the (positive) photoresistcomposition is applied on a substrate such as a silicon wafer, and isthen dried to thereby form a photosensitive layer; and thephotosensitive layer is exposed to light through a patterned photomask.

[0088] Next, the exposed portions of the photosensitive layer aresubjected to post-exposure baking (PEB) and are then dissolved andremoved by dipping the substrate in a developer solution, for example,an alkaline aqueous solution such as a 1% to 10% by weighttetramethylammonium hydroxide (TMAH) aqueous solution, thus forming animage (resist pattern) being in exact accordance with the mask pattern.To further improve the definition of the resist pattern, ananti-reflection coating is preferably formed between the substrate andthe photosensitive layer composed of the invented composition.

[0089] [Process for Forming Resist Pattern with High Aspect Ratio]

[0090] The invented photoresist compositions are specifically suitablefor the process for the formation of a resist pattern using a silylationagent. In the process, adjacent resist patterns (line width) areenlarged and grooves (space width) are narrowed by using a silylationagent to thereby form a resist pattern with a high aspect ratio, and theinvented photoresist compositions can be advantageously used in thisprocess. The process will be schematically illustrated below, but theinvention is not limited thereto.

[0091] (Step a)

[0092] A first resist that can be smoothed is applied on a substrate andis dried to thereby form a first resist layer (herein after referred toas “bottom resist”, which will be described later), and the inventedphotoresist composition that can form a resist pattern byphotolithography is applied onto the first resist layer and is dried tothereby form a second resist layer (hereinafter referred to as “topresist”).

[0093] (Step b)

[0094] Subsequently, the top resist is exposed to imaging radiation andis then subjected to a heat treatment, and the exposed portions (in caseof a positive photoresist composition) or unexposed portions (in case ofa negative photoresist composition) are dissolved and removed bydeveloping in an alkaline aqueous solution to thereby form a resistpattern in exact accordance with the mask pattern.

[0095] (Step c)

[0096] Next, a silylation agent, which will be described later, isapplied onto the formed resist pattern and the resist pattern is rinsedto enlarge the resist pattern (line width) and to form a silylationcoating. The silylation coating is resistant to corrosion induced byoxygen-containing plasma etching.

[0097] (Step d)

[0098] The groove (space pattern) or hole (hole pattern) is narrowed byenlargement of the resist pattern carrying the silylation coating, andthe bottom resist is etched with oxygen-containing plasma using thenarrowed groove or hole as a mask to thereby form a resist pattern witha desired height in the bottom resist.

[0099] This process can therefore form a resist pattern with a higheraspect ratio.

[0100] [Bottom Resist]

[0101] In the above process, the bottom resist is preferably composed ofa material resistant to etching of the underlayer substrate, and is morepreferably composed of a novolak resin. As such materials, photoresistcompositions mainly containing a novolak resin and a quinonediazide aresuitable. By heating the applied bottom resist on the substrate,cross-linking of the novolak resin occurs to thereby improve resistanceto etching and to inhibit dissolution of the bottom resist and the topresist (the invented photoresist composition) in each other.

[0102] [Silylation Agent]

[0103] Silylation agents for use in the above process are enlargingreagents that can be bonded with the resist pattern of the top resist(the invented photoresist composition) and can increase the volume ofthe resist pattern. Additionally, the resist pattern carrying asilylation coating composed of the silylation agent can become a layerthat is very resistant to corrosion induced by dry developing inoxygen-containing plasma etching for etching the bottom resist.

[0104] Silylation agents having a Si-O (siloxane) structure arepreferred as the silylation agents, since a nonvolatile oxide is formedfrom a silicon-containing group in the silylation coating duringoxygen-containing plasma etching to thereby enhance corrosionresistance, and the siloxane compounds are highly thermally stable tothereby avoid pattern deformation during the etching step at hightemperatures. These siloxane compounds can be easily directly convertedinto silicon dioxide and produce little volatile organosilicon compoundin oxygen-containing plasma. Among them, siloxane compounds having anamino group are preferred, such as aminosiloxane, diaminosiloxane, andbisaminopropylpolydimethylsiloxane. The amino group in the silylationagent rapidly reacts with Ingredient (B) of the copolymer in theinvented photoresist composition on the surface of the resist pattern ofthe top resist to increase the volume of the resist pattern to therebynarrow the space pattern.

[0105] Commercially available agents can be used as the silylationagents. For example, a preferred silylation agent solution for use inthe invention is a 1% by weight bisaminopropylpolydimethylsiloxanesolution prepared by dissolving bisaminopropylpolydimethylsiloxane(available from Shin-Etsu Chemical Co., Ltd., under the trade name of“X-22-161 AS”) in an aqueous 1-hexanol solution.

BRIEF DESCRIPTION OF THE DRAWINGS

[0106]FIG. 1 is a diagram showing the spectrum of Copolymer 1 obtainedin Synthesis Example 1, measured by Fourier transform infraredspectrophotometry (FT-IR); and

[0107]FIG. 2 is a diagram showing the chart of Copolymer 1 obtained inSynthesis Example 1, measured by gel permeation chromatography (GPC).

EXAMPLES

[0108] The present invention will be illustrated in further detail withreference to several examples and comparative examples below, which arenot intended to limit the scope of the invention.

[0109] The physical properties of the photoresist compositions weredetermined according to the following methods.

[0110] (1) Sensitivity Evaluation

[0111] Initially, a novolak resin-quinonediazide positive photoresistcomposition (available from Tokyo Ohka Kogyo Co., Ltd., under the tradename of “THMR-iP5700) was applied on a silicon wafer, and was cured byheat to thereby form a resist film (bottom resist) 0.6 μm thick. Thiswas used as a substrate in the subsequent procedure. A sample wasapplied onto the substrate using a spinner, and was dried on a hot plateat 110° C. for 90 sec. to form a resist film 0.2 μm thick. The resistfilm was then irradiated through a mask corresponding to a hole diameterof 0.23 μm and a duty ratio of 1:0.57 (interval between holes: 0.13 μm)using a reducing-type projection aligner NSR-S203B (available from NikonCorporation, Japan; NA=0.60, δ=0.68) with increasing bias from 10 mJ/cm²at intervals of 1 mJ/cm². The film was then post-exposure baked (PEB) at130° C. for 90 sec.; was subjected to developing in a 2.38% by weighttetramethylammonium hydroxide aqueous solution at 23° C. for 40 sec.,was washed with water for 15 sec., and was dried. In this procedure, thesensitivity was defined as the exposure time period (Eop) (mJ/cm²) toreproduce a 0.18-tm hole pattern.

[0112] (2) Definition

[0113] The definition was defined as the minimum mask size (μm) in thecritical definition to separate the bottom of the pattern at an exposureof Eop in the procedure of Evaluation (1).

[0114] (3) Focal Depth Range Properties

[0115] A sample was subjected to exposure and developing in the samemanner as in Evaluation (1), except that the focus was shifted up anddown at an exposure of Eop as the standard exposure. The resulting holepattern was subjected to a scanning electron micrographic (SEM)observation. Based upon the SEM photograph, the focal depth rangeproperty was defined as the maximum value (am) of the focal shift(defocus) to reproduce a 0.18-μm hole pattern when the hole pattern wasformed under bias using a 0.23-μm mask.

Synthesis Example 1

[0116] Synthesis of Copolymer 1

[0117] To 69.4 parts by weight of ethyl acetate in a reactor, 15.9 partsby weight (0.36 mole) of 2-methyl-2-adamantyl methacrylate, 9.9 parts byweight (0.54 mole) of maleic anhydride, 6.4 parts by weight (0.30 mole)of allyltrimethylsilane, and 0.6 part by weight ofazobisisobutyronitrile were added, and were stirred at room temperature(25° C.) for 60 minutes.

[0118] Next, the resulting mixture was heated to about 70° C. and wasstirred at this temperature for about 22 hours.

[0119] To the resulting reaction mixture, 11.5 g (0.36 mole) of a 2.0%by weight methanol aqueous solution was added dropwise, and the mixturewas further stirred for about 22 hours.

[0120] After the completion of stirring, the heating operation wasstopped, and the reaction mixture was cooled to about 30° C. and wasthen added dropwise to petroleum benzine with stirring to therebyprecipitate a copolymer.

[0121] The precipitated copolymer was rinsed several portions ofpetroleum benzine and was dried in a vacuum dessicator set at about 60°C. for 3 hours to thereby yield Copolymer 1.

[0122] Copolymer 1 was subjected to FT-IR assay by the KBr method usingan FT-IR system (available from The Perkin-Elmer Corporation, under thetrade name of “SPECTRUM 2000”).

[0123] The results are shown in FIG. 1.

[0124]FIG. 1 shows that the characteristic band of adamantyl moiety of2-methyl-2-adamantyl methacrylate in the vicinity of 1100 cm⁻¹, theabsorption of C═O stretching vibration of maleic anhydride in thevicinity of 1780 cm⁻¹, and the absorption of the deformation vibrationof Si—CH₃ of allyltrimethylsilane in the vicinity of 1250 cm⁻¹ wererespectively observed.

[0125] Separately, Copolymer 1 was subjected to gel permeationchromatographic (GPC) analysis by the R.I. method using a GPC system(available from Shodex, under the trade name of “GPC-SYSTEM-11”).

[0126] The results are shown in FIG. 2.

[0127]FIG. 2 shows that Copolymer 1 was a polymer having a weightaverage molecular weight (Mw) in terms of polystyrene of about 18700 anda molecular-weight distribution (Mw/Mn) of about 1.67.

Synthesis Examples 2 to 4

[0128] Synthesis of Copolymers 2 to 4

[0129] Copolymers 2 to 4 containing the repeating units indicated inTable 1 were synthetically obtained in the same manner as in SynthesisExample 1, except that the types and compositional ratios of individualcomonomers were varied.

Comparative Synthesis Examples 1 and 2

[0130] Synthesis of Copolymers 5 and 6

[0131] Copolymers 5 and 6 containing the repeating units indicated inTable 1 were synthetically obtained in the same manner as in SynthesisExample 1, except that the types and compositional ratios of individualcomonomers were varied. Copolymer 5 as a comparative example wasobtained by using tert-butyl methacrylate as a comonomer. The repeatingunit derived from tert-butyl methacrylate is a unit represented byFormula (X-1) below. TABLE 1 (X-1)

Example No. (Copolymer) (B) (C) (D) (X) (MW) (Mw/Mn) (mole %) (mole %)(mole %) (mole %) Synthesis Example 1 B-2 C D-1 none (Copolymer 1) (45)(25) (30) (18700) (1.67) Synthesis Example 2 B-3 C D-1 none (Copolymer2) (45) (25) (30) (10000) (1.6) Synthesis Example 3 B-2 C D-1 none(Copolymer 3) (45) (20) (35) (15000) (1.9) Synthesis Example 4 B-2 C D-1none (Copolymer 4) (45) (30) (25) (16800) (1.9) Comp. Syn. Ex. 1 B-2 Cnone X-1 (Copolymer 5) (50) (27.5) (22.5) (20000) (2.0) Comp. Syn. Ex. 2B-2 none D-1 none (Copolymer 6) (70) (30) (20000) (2.0)

Examples 1 to 4 and Comparative Examples 1 and 2

[0132] Each of Copolymers 1 to 6 synthetically obtained in SynthesisExamples 1 to 4 and Comparative Synthesis Examples 1 and 2 was dissolvedin PGMEA to thereby yield a 7% by weight polymer solution.

[0133] In 30 g of the above-prepared polymer solution, 0.158 g of atriphenylsulfonium-based onium salt represented by Formula (E-3) belowwas dissolved, and the resulting solution was filtrated though amembrane filter of 0.1 to 0.2 μm pore size to thereby yield a series ofphotoresist compositions.

[0134] Evaluations (1) to (3) were conducted on the photoresistcompositions prepared according to Examples 1 to 4 and ComparativeExamples 1 and 2, and the results are shown in Table 2. TABLE 2Sensitivity Definition Focal depth range (mJ/cm²) (μm) property (μm)Example 1 45 0.16 0.6 Example 2 45 0.17 0.6 Example 3 30 0.16 0.5Example 4 55 0.17 0.5 Comp. Ex. 1 40 0.18 0.6 Comp. Ex. 2 80 0.18 0.4

Synthesis Example 5

[0135] Preparation of Silylation Agent Solution

[0136] Bisaminopropylpolydimethylsiloxane (available from Shin-EtsuChemical Co., Ltd., under the trade name of “X-22-161 AS”) was dissolvedin a 98.5% by weight 1-hexanol aqueous solution to thereby yield a 1% byweight silylation agent solution.

Example 5

[0137] Formation of a Resist Pattern with a High Aspect Ratio using theSilylation Agent

[0138] A hole pattern [a hole pattern 0.18 μm wide] was formed on asubstrate at depth of focus of 0 in the same manner as in Evaluation (3)in Example 1, and the above-prepared silylation agent solution wascarefully placed as a heap on the substrate carrying the hole pattern,and was allowed to stand for 30 seconds, and the substrate was rinsedwith isopropanol for 15 seconds.

[0139] The cross section (profile) of the resist pattern after thesilylation was subjected to SEM photographic observation and was foundthat the initial hole diameter, 0.18 μm, was narrowed to 0.14 μm by theformation of a silylation coating and that the hole profile was notdeteriorated.

Comparative Example 3

[0140] Formation of a Resist Pattern with a High Aspect Ratio using theSilylation Agent

[0141] A hole pattern [a hole pattern 0.18 μm wide] was formed on asubstrate at depth of focus of 0 in the same manner as in Evaluation (3)in Comparative Example 1, and the above-prepared silylation agentsolution was carefully placed as a heap on the substrate carrying thehole pattern, and was allowed to stand for 30 seconds, and the substratewas rinsed with isopropanol for 15 seconds.

[0142] The cross section (profile) of the resist pattern after thesilylation was subjected to SEM photographic observation and was foundthat the initial hole diameter, 0.18 μm, was narrowed to 0.15 μm by theformation of the silylation coating but that the hole profile wasdeteriorated.

Comparative Example 4

[0143] Formation of a Resist Pattern with a High Aspect Ratio using theSilylation Agent

[0144] A hole pattern [a hole pattern 0.18 μm wide] was formed on asubstrate at depth of focus of 0 in the same manner as in Evaluation (3)in Comparative Example 2, and the above-prepared silylation agentsolution was carefully placed as a heap on the substrate carrying thehole pattern, and was allowed to stand for 30 seconds, and the substratewas rinsed with isopropanol for 15 seconds.

[0145] The cross section (profile) of the resist pattern after thesilylation was subjected to SEM photographic observation and was foundthat no silylation coating was formed and that the hole diameter was notnarrowed.

[0146] Advantages

[0147] The present invention can provide a chemically amplifiedphotoresist composition that has satisfactory transparency, highsensitivity and definition in the field of photolithography using a deepUV light source such as KrF or ArF laser, and provide a novel copolymerthat is suitable for the preparation of the photoresist composition.

[0148] Additionally, the present invention can provide a chemicallyamplified photoresist composition that is suitable for a process for theformation of a resist pattern with a high aspect ratio using asilylation agent, and provide a process for the formation of a resistpattern with a high aspect ratio.

[0149] Other embodiments and variations will be obvious to those skilledin the art, and this invention is not to be limited to the specificmatters stated above.

What is claimed is:
 1. A novel copolymer comprising: a repeating unit(B) derived from an unsaturated carboxylic anhydride; a repeating unit(C) represented by Formula (II); and a repeating unit (D) represented byFormula (III):

wherein R¹ is a hydrogen atom or a methyl group; and R² is an alkylgroup having from 1 to 4 carbon atoms.
 2. A novel copolymer according toclaim 1, wherein said repeating unit (B) is a repeating unit (B-1)derived from a unsaturated cyclic carboxylic anhydride.
 3. A novelcopolymer according to claim 2, wherein said repeating unit (B-1) is aunit (B-2) represented by Formula (V):


4. A novel copolymer according to claim 2, wherein said repeating unit(B-1) is a unit (B-3) represented by Formula (XVI):


5. A novel copolymer according to claim 1, wherein said repeating unit(D) is a unit (D-1) represented by Formula (VI):


6. A novel copolymer according to claim 1, wherein the content ofrepeating unit (B) is equal to or more than 15% and equal to or lessthan 60% of all repeating units constituting said novel copolymer.
 7. Anovel copolymer according to claim 1, wherein the content of repeatingunit (C) is equal to or more than 10% and equal to or less than 40% ofall repeating units constituting said novel copolymer.
 8. A novelcopolymer according to claim 1, wherein the content of repeating unit(D) is more than 0% and equal to or less than 40% of all repeating unitsconstituting said novel copolymer.
 9. A novel copolymer according to anyone of claims 1 to 8, wherein said novel copolymer has a weight averagemolecular weight (Mw) in terms of polystyrene of from 7000 to 30000 anda molecular-weight distribution (Mw/Mn, where Mn is a number averagemolecular weight) of equal to or less than 3.5.
 10. A photoresistcomposition comprising: a novel copolymer according to claim 1; aphotosensitive acid generator; and an organic solvent.
 11. A photoresistcomposition according to claim 10, wherein said photosensitive acidgenerator is a triphenylsulfonium-based onium salt.
 12. A photoresistcomposition according to claim 10, wherein said organic solvent ispropylene glycol monomethyl ether acetate (PGMEA).
 13. A process forforming a resist pattern with a high aspect ratio, said processcomprising the steps of: (a) applying a first resist on a substrate anddrying the applied first resist to thereby form a first resist layer,applying a photoresist composition of claim 10 onto the first resistlayer and drying the applied photoresist composition to thereby form asecond resist layer; (b) exposing the second resist layer to imagingradiation, subjecting the exposed second resist layer to a heattreatment, and dissolving and removing exposed portions or unexposedportions of the second resist layer by developing in an alkaline aqueoussolution to thereby form a resist pattern; (c) applying a silylationagent onto the formed resist pattern, rinsing the applied resist patternto thereby enlarge the resist pattern and to form a silylation coatingon the resist pattern, said silylation coating being resistant tocorrosion induced by oxygen-containing plasma etching; and (d) etchingthe first resist layer under the second resist layer withoxygen-containing plasma by using, as a mask, the enlarged resistpattern carrying the silylation coating.